Automobile seatbelts, like airbags, were designed for adults. Infant carriers and car seats protect infants and children up to approximately four years.
At four a child has outgrown her car seat, but use of an adult seatbelt alone can produce life-threatening abdominal and spinal injuries.
The current solution for children 4 to 10 year of age is the belt-positioning booster seat. It elevates the child to position the adult seatbelts to mimic their paths across the body of an adult, with the lap seatbelt passing across the spine on either side of the child's pelvis.
The technique has resulted in a preoccupation with the static fit of the lap seatbelt, disregarding the unlikelihood the static belt fit will be preserved in a collision.
The most egregious aspect of the preoccupation with static fit is the failure to protect the child against the lap seatbelt itself.
In a collision the lap seatbelt can slip over the child's pelvis, allowing her to slip forward and ‘submarine’ under the belt, with the brunt of the collision forces focused on the narrow band of seatbelt across her vulnerable abdomen. She is thus at risk of lap seatbelt-inflicted internal trauma including the evisceration of internal organs and the stretching and permanent rupture of the spinal cord due to the bending of the lumbar spine about the seatbelt acting as a fulcrum.
Proof of the relative ineffectiveness of belt-positioning booster seats is provided by a National Highway Traffic Safety Administration (NHTSA) study published in July 2010 which reported a 14-percent reduction in overall injuries for children in booster seats relative to children in adult seatbelts alone.
This modest statistical margin of safety is largely due to increased use of the shoulder belt rather than any inherent quality of the belt-positioning booster seat itself. It is offset by a large increase in the incidence of head injuries with use of the belt-positioning booster seat due to the child's seating position being both elevated and advanced in order to achieve a proper ‘static’ belt fit.
Our understanding of the mechanics of automobile collisions has evolved over the thirty years since the invention of the booster seat, and passenger vehicles have gone from having a body on a rigid frame to a unibody construction, designed to absorb the energy of a collision by a progressive collapse.
To be most effective, restraints must exploit this advance in automotive design and couple passengers to the vehicle as tightly as possible to ‘ride down’ a crash with the vehicle to maximize the benefit of its energy-absorbing capacity.
The benefit is lost to the extent to which the passengers are free to move in a collision in advance of their own impact with either the lap seatbelt or the interior of the vehicle, whether it is due to the design of the restraint, its interaction with the vehicle interior or its misuse. Excessive free movement in a collision is an inevitable by-product of the design of the belt positioning booster seat.
By both elevating and advancing the child, the belt positioning booster seat extends the length of the lap seatbelts. In a collision the lap seatbelts rotate clockwise about their anchors with compression of the vehicle seat, degrading static fit, and exposing the fallacy of static lap belt fit.
A majority of users of belt-positioning booster seats misuse them misrouting one or both belts, accepting a child's poor posture or not adjusting the lap seat belt snug about the child. Almost all cases of misuse result in excessive belt slack, increasing forward movement which, in addition to further degrading the static fit, results in sharp increases in pelvic loading by the belt.
The focus on static belt fit is based on an assumption that a child's pelvis is a scaled-down version of an adult pelvis; in reality the pelvis of a child is not well suited to either locating the belt, being proportionally smaller and lower profile than an adult pelvis, or assuming the brunt of collision forces concentrated in the lap belt entirely on its own, being composed significantly of elastic cartilage.
Maintaining proper pelvic orientation is thus essential in a collision: when a child restrained by a lap seatbelt slouches, her pelvis undergoes a counter-clockwise rotation, reducing the height and decreasing the vertical slope of the forward edges of the iliac spines on either side of the pelvis, thus diminishing the capacity of the spines to engage with either a seatbelt or restraint to absorb collision forces and resist submarining.
The forward edge of either iliac spine may be viewed as a ‘ramp’, whereby the outcome of a collision, in terms of whether a seatbelt or restraint is either held captive in front of the pelvis as in FIG. 11, or rides up the ‘ramps’ to allow the pelvis to rotate and the child to submarine, is a consequence of:                (i) the magnitude of the downward reaction force (Rθ) FIG. 7 exerted by the belt or restraint on the hips relative to        (ii) the inertial force of the child's lower body tending to make the child slip forwards under the belt and        (iii) the angle of the ‘ramps’.        
Despite the emphasis on static lap seatbelt fit with its implicit recognition of the risk of lap seatbelt-inflicted internal trauma, the belt-positioning booster seat makes no provision to protect the child against the lap seatbelt in a collision.
Thus it is imperative with a belt-positioning booster seat that the shoulder belt protect the child against the lap seatbelt by sharing the load of the collision forces.
Both backless and high-back belt-positioning booster seats increase the likelihood of a proper shoulder seatbelt fit in a collision, but a proper shoulder belt static fit does not guarantee proper routing under dynamic collision conditions.
Where the fit of a shoulder belt is dependent upon a plastic feature of a high-back booster seat there is no guarantee that collision forces will not make the belt revert to the original path determined by the seatbelt geometry of the vehicle.
Preserving shoulder belt fit in a collision is essential to protecting the child against the lap seatbelt; it is not a guarantee a child will not submarine or be otherwise injured by the lap belt.
Although there is a correlation between static lap seatbelt fit and belt angle, it is the angle assumed by the lap seatbelt when the child and her restraint are thrown forward in a collision that is determinant of the outcome.
Lap Seatbelt Geometry: Vertical Angle
A lap seatbelt restrains an adult passenger by exerting reaction forces on her body both in direct opposition to forward collision forces and downward into the vehicle seat.
In a collision FIG. 7 the force of the inertia (I) of a passenger's body acting on the lap seatbelt translates into a tension (T) and a downward reaction force (Rθ) in the lap seatbelt.
This downward reaction force is essential to ensuring the belt does not ride up over the pelvic spines to penetrate the soft abdomen.
The downward reaction force in the lap seat belt is proportional to the angle (θ) between the seat belt and the horizontal collision forces and is counteracted by a reaction force (RN) normal to the surface of the seat, resulting in a proportional friction force (FRN) opposing the tendency to slide forward.
The collision force acting horizontally on the pelvis due to the inertia of the legs is thus reduced by the gripping action of the seatbelt or restraint on the thighs and the frictional force FRN.
The performance of a lap seatbelt or restraint is thus dependent upon the angle (θXZ) the belt assumes FIG. 5A in the X-Z plane in relation to the horizontal when subjected to collision forces, and that angle is determined by the point of contact of the lap seatbelt on the passenger relative to the location of the seatbelt anchors.
The steeper the angle θ of the belt, the greater is the force Rθ exerted downwards on the hips and upper thighs as is the corresponding frictional force FRN.
In a collision a passenger moves forward and downwards with compression of the vehicle seat by the downward reaction force in the lap seatbelt and there is a corresponding clockwise rotation of the lap seatbelts about their anchors, resulting in a shallower angle θ.
Too shallow an angle and there is not enough downward pressure in the belt Rθ on the hips and a risk inertial forces will make the passenger submarine, exposing the abdomen to the lap seatbelt.
The child passenger is more dependent upon the preservation of a taller belt angle to resist the tendency to submarine than an adult because her pelvis lacks the height, definition and integrity to reliably ‘hook’ the belt.
The pelvis of a child is also unable to assume the brunt of the forces of a collision on its own. For the pelvis to play its role in protecting the viscera of the child, it depends upon sufficient downward reaction force in the lap seatbelt to grip the upper thighs, augmenting the friction forces opposing her tendency to slide forward and the pelvis to rotate counter-clockwise.
By elevating and advancing the child, belt-positioning booster seats extend the lap seatbelts, increasing free forward movement in a collision, degrading belt angle and reducing the magnitude of the downward reaction force essential to stabilizing the pelvis.
Lap Seatbelt Geometry: Adjustment
A snug lap seatbelt adjustment is essential to maintaining proper pelvic orientation and limiting the loads imposed on the pelvis in a collision. In a collision the child will continue to travel at the original velocity of the vehicle, heedless of its deceleration, until she collides with a seatbelt, child restraint or the vehicle interior, producing a spike in the loading of the pelvis.
A restraint should minimize this useless movement by coupling the child to the vehicle as tightly as possible so as to ‘ride down’ the crash with the vehicle to the fullest extent possible, participating in its more gradual deceleration due to its capacity to absorb the energy of the collision.
A snug seatbelt adjustment is more easily achieved the more closely the lap belts are aligned (in the X-Y plane) with the direction of the collision forces (the greater is angle θxy) FIGS. 5B, 6B.
Ideally lap seatbelt anchor points should be no further apart than the pelvic width of the passenger in order to achieve a tight lap seatbelt adjustment; a four year old in a vehicle is at a disadvantage with a pelvic width half that of an adult.
A snug lap seatbelt is particularly important in a sports utility vehicle (SUV) which does not have the same collision energy absorbing capacity as a passenger car by virtue of its rigid chassis construction.